The invention relates to an arrangement for generating X-rays upon incidence of electrons, which arrangement includes a liquid metal zone in which a liquid metal is provided as an X-ray target in such a manner that it can flow past a zone of electron incidence. The invention also relates to an X-ray source which includes an electron source for the emission of electrons and such an arrangement for generating X-rays. Finally, the invention also relates to an X-ray device which includes an X-ray detector and such an X-ray source.
An arrangement and an X-ray source of this kind are known from U.S. Pat. No. 6,185,277 B1. Therein, the electrons emitted by an electron source penetrate the liquid metal through a thin window and generate X-rays therein. The liquid metal, having a high atomic number, circulates therein under the influence of a pump, so that the heat produced by the interaction with the electrons in the window and in the liquid metal can be dissipated. The heat produced in this zone is carried off by a turbulent flow, thus ensuring effective cooling.
A number of different applications is feasible for such an arrangement for generating X-rays. In the case of a computed tomography apparatus, an X-ray source is required which is capable of delivering a high pulsed power of, for example, approximately 80 kW for a brief period of time only, for example, for approximately 20 s. For a different type of application, that is, in an X-ray system for the inspection of luggage, for example, for the presence of explosives or drugs, however, a lower power of only, for example, approximately 30 kW is required, be it that this power has to be delivered continuously, that is, for several hours.
For the known X-ray source provided with a liquid metal target, in which the liquid metal is circulated by means of a pump, it has since long been assumed that the described conditions can be satisfied by means of a single pump. However, it has been found that for the first type of application, that is, in computed tomography, the required pulsed power is very high, but the mean power is much lower. Assuming that each period of use of approximately 20 s is typically preceded by an idle time of approximately 80 s, the mean electric power then amounts to (80 kW.20 s)/(80 s+20 s)=16 kW. Consequently, it should also be possible to reduce the power of the pump accordingly, that is, to conceive the pump for the required mean power of approximately 16 kW instead of for the maximum pulsed power of 80 kW; this would offer a significant saving in respect of space and costs.
The present invention, therefore, has for its object to provide an arrangement for generating X-rays which is provided with a liquid metal target and can be used for various applications and requires only a comparatively small pumping capacity for the liquid metal. On the basis of the arrangement of the kind set forth, this object is achieved in that a pressure zone which is separate from the liquid metal zone is provided with a pressure medium in such a manner that the pressure medium can exert a pressure on the liquid metal present in the liquid metal zone in order to force the liquid metal past the zone of electron incidence, the pressure zone being provided with a pressure accumulator which can be replenished in order to apply the pressure.
In accordance with the invention it was recognized that the pumping capacity required for forcing the liquid metal past the zone of electron incidence need not be conceived for the highest electric (pulsed) power, but that the required pumping capacity can be tuned to the mean electric power required when supplementary means are provided for the storage of pumping capacity. Assuming that the energy required to move a liquid volume V through a pressure difference amounting to xcex94P equals V.xcex94P, the pump requires a capacity of 1/xcex5.(V.xcex94P)/T. The value xcex5 takes into account the fact that the conversion of mechanical energy into hydrodynamic energy has an efficiency of less than 100%; T is the period of time over which the energy transfer to the liquid metal can be distributed. The pumping capacity can thus be significantly reduced by distributing the supply of energy in the form of pumping energy over 100 s (in the above example concerning computed tomography) instead of concentrating it to 20 s only.
Thus, in accordance with the invention the following conditions must be satisfied:
a) the energy for driving the liquid metal can be effectively stored, replenished and extracted in a short period of time whenever necessary;
b) the type of energy storage is compatible with the condition that the liquid metal must be driven in a pump-like fashion.
The foregoing is achieved in accordance with the invention in that, unlike in the known X-ray source, the liquid metal is not circulated by means of a pump but is situated exclusively in a liquid metal zone in which it can be moved to and fro, however, without being circulated. Furthermore, there is provided a pressure zone which is separate therefrom and also includes a pressure accumulator in which energy can be stored so as to be extracted for moving the liquid metal in the liquid metal zone with the desired power, that is, for conducting the liquid metal past the zone of electron incidence. In order to recharge the pressure accumulator, that is, to replenish energy, there may be provided a recharging device, for example, a pump which has a capacity which is significantly lower than that of the pump in the known X-ray source, because the energy in the pressure accumulator can now be dispensed at any time, that is, also during the idle periods of the X-ray source, whereas in the known X-ray source the full pumping capacity must be made available during operation. Such a recharging device, therefore, can be constructed so as to save room and money and also enables universal use of such an X-ray source.
Preferably, the liquid metal zone and the pressure zone adjoin one another in two locations, that is, in two so-called separation zones, in which a pressure can be exerted on the liquid metal by means of the pressure medium. Such separation zones may be conceived, for example, as respective separating chambers with a liquid metal chamber and a pressure medium chamber each, the liquid metal and the pressure medium being separated by a flexible diaphragm via which the pressure can be transferred from the pressure medium to the liquid metal. The liquid metal as well as the pressure medium can thus expand into the relevant separating chamber as a function of the adjusted pressure ratio.
Other solutions are also feasible for the conception of the liquid metal zone and the pressure zone. However, it is a common aspect of all such solutions that a pressure is exerted indirectly, via the pressure medium, on the liquid metal in the liquid metal zone, so that the liquid metal is not driven directly. The separation zones may thus also be construed as a cylinder with a respective displaceable piston, where the piston serves as a separating means between the liquid metal and the pressure medium; it can in principle also be constructed so as to be driven in an arbitrary manner.
Various alternatives for the pressure medium are disclosed in which a gas, notably air, is preferably used as the pressure medium.
For the control of the application of pressure to the liquid metal, and hence for the control of the flow velocity of the liquid metal in the zone of electron incidence, there are provided appropriate control means. These control means may notably be provided with the previously mentioned controllable valves via which the application of pressure from the pressure accumulator to the liquid metal zone, notably to the separating zones, can be controlled.
In order to achieve an as high as possible flow velocity in the zone of electron incidence, the liquid metal zone may be provided with a constriction at the area of said zone of incidence. This constriction may be conceived so as to be asymmetrical to both sides of the zone of electron incidence, for example, so as to approach the external shape of a drop of water, so that the loss of pressure incurred by the liquid metal flowing through the constriction is as small as possible. However, in that case it is necessary to take into account the fact that during operation the liquid metal should always flow through the constriction in one direction only in order to ensure that the greatest possible desired effect is achieved.
During operation the liquid metal is heated up to a few 100xc2x0 C. In order to cool the heated liquid metal, therefore, cooling means, for example, in the form of cooling ducts which extend around the separating zone, are provided in at least one of the two separating zones in which the liquid metal is preferably present after a period of use.
The arrangement for generating X-rays preferably forms part of an X-ray source which includes an electron source for the emission of electrons. Such an X-ray source is preferably used in conjunction with an X-ray detector in an X-ray device.
The following description, claims and accompanying drawings set forth certain illustrative embodiments applying various principles of the present invention. It is to be appreciated that different embodiments applying principles of the invention may take form in various components, steps and arrangements of components and steps. These described embodiments being indicative of but a few of the various ways in which some or all of the principles of the invention may be employed in a method or apparatus. The drawings are only for the purpose of illustrating an embodiment of an apparatus and method applying principles of the present invention and are not to be construed as limiting the present invention.